A question though, how do you remove instruments and patterns(arrangements)?

Though they do not effect the sounds and patterns, what so ever, they do take up UI space and makes navigating both the instruments and patterns more difficult.

A possible "conflict" in the removal of an instrument is the case where targeted instrument is in-use. You could probably handle this by defaulting all patterns using the instrument (no longer available) to the another instrument (possibly the instrument in slot one).

Another possible "conflict" is the removal of all instruments, which can be dealt with by defaulting to the piano instrument.

In the patterns case, you ought to simply remove the pattern from arrangement if they are no longer available; a lot more simpler (maybe).

And while we're at it, though not necessary, rearrangements of instruments and patterns would be cool. I haven't taken a look at the source yet, but hopefully shifting data up and down isn't too difficult.

But even without the above, the application still works. It's just tedious to explain the partners that those were instruments or patterns that were once consider but abandoned and could not be removed from the list (And that brings up aliasing for the list... guess that would be asking for too much).

I just had the very same question (removing patterns) and after some experimenting I found out that you can remove pattern by dragging them into the lower-right corner of the tool. Simply click and hold on an entry in the patterns list, then the delete button appears in the bottom-right and you can drag it onto that button to delete it.